The ceremony will draw tens of thousands to honor the tiny, stooped nun who was fast-tracked for sainthood just a year after she died in 1997. St. John Paul II, who was Mother Teresa's greatest champion, beatified her before a crowd of 300,000 in St. Peter's Square in 2003.

Francis, whose papacy has been dedicated to ministering to the poor just as Mother Teresa did, is a known fan. During his September 2014 visit to Albania, Francis confided to his interpreter that he was not only impressed by her fortitude, but in some ways feared it.

Francis recounted that he had met Mother Teresa, an ethnic Albanian, when they attended a 1994 bishop synod at the Vatican together. At the time, he was Archbishop Jorge Mario Bergoglio.

"Bergoglio had Mother Teresa behind him, nearby, and he heard her intervene often with great strength, without letting herself in any way be intimidated by this assembly of bishops," the Vatican spokesman, the Rev. Federico Lombardi, later recounted. "And from that he developed a great esteem for her, as a strong woman, a woman able to give courageous testimony."

But Bergoglio, who has long shown admiration for the women who raised him and taught him, added: "I would have been afraid to have had her as my superior, since she was so tough."

Mother Teresa blessing Sister Nirmala, the new head of the Missionaries of Charity at Mother Teresa's house in Calcutta.

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Born Agnes Gonxha Bojaxhiu on Aug. 26, 1910, in Skopje, Macedonia, Mother Teresa joined the Loreto order of nuns in 1928. In 1946, while traveling by train from Calcutta to Darjeeling, she was inspired to found the Missionaries of Charity order.

The order was established four years later and has since opened more than 130 houses worldwide to provide comfort and care for the needy, dying, sick and "poorest of the poor."

Mother Teresa won the Nobel Peace Prize in 1979 for her work with Calcutta's destitute and ill - work which continued even after she herself became sick. She died on Sept. 5, 1997, at age 87. At the time, her Missionaries of Charity order had nearly 4,000 nuns and ran roughly 600 orphanages, soup kitchens, homeless shelters and clinics.

"She built an empire of charity," said the Rev. Bernardo Cervellera, editor of the Vatican-affiliated missionary news agency AsiaNews. "She didn't have a plan to conquer the world. Her idea was to be obedient to God."